A method for the calibration of the color printing process mentioned above is shown in U.S. patent application Ser. No. 10/681,849, filed on Oct. 8, 2003. Furthermore, a method for determining register errors has been described in U.S. patent application Ser. No. 10/208,216, filed on Jul. 30, 2002.
It is known that contingent on the print substrate itself, register mark errors that is, errors in the precise stacked printing of the color separations, may occur in the case of color printing, and may influence particularly the circumference register, or the so-called intrack register in the direction of the printing process. As a result, register mark errors, i.e. errors in the precise positioning of a print layout on a printed sheet, may also occur particularly in the circumference register within the color or monochrome printing process. The terms “register” or “registration”, also are used in the following in the broader sense as the generic terms, including the register mark. Conversely, the explanations provided only for the register mark analogously are often also true for the register.
In the case of print substrate conditional register mark errors, i.e. errors which adversely affect the registration of the color separation processes of a color print relative to each other, and individually to the substrate to be printed, particularly the so-called gear register mark error, the so-called creep register mark error, and the so-called print substrate conditional magnification register mark error, which should not be confused with the periodic magnification error caused by a concentricity error, can be differentiated.
The gear register mark error can be caused by the fact that a print substrate is forced through the narrow gap (nip) between the conveyor belt and a printing organ or transfer organ (for example, a rubber blanket cylinder), respectively; the print substrate is driven by the movement and transfers the print layout onto the substrate to be printed, the speed of which is thereby changed (as, for example, in a gear drive by the change of the transmission ratio).
The creep register mark error can be caused by the fact that the surface of the substrate (so to speak ranking on a curved path) to be printed in a re-directional range, or otherwise curved path of the conveyor belt, is positioned on a different radius toward the re-directional or curvature axis than the surface of the conveyor belt and, therefore, has a different path speed.
The print substrate conditional magnification register mark error can be caused by the fact that a print substrate is forced through the narrow gap (nip) between the conveyor belt and printing organ or transfer organ, respectively, transferring the print layout onto the substrate to be printed, the shape of which is thereby changed, which may lead to the spreading or enlargement of the print layout in the running direction of the print substrate.
The previous paragraphs each list which cause may be responsible for the respective register mark error, because the causes may be very complex and, ultimately, are not safely and completely clarified. For example, temperature or moisture fluctuations may also be responsible for the register mark error. In particular, however, register mark errors turn out to be of various sizes depending on the print substrate, for instance, depending on the format, thickness, coating, history, etc. Therefore, any correction performed online is particularly desired. Always important, however, is the correction of the error, regardless of whether the cause of the error has been completely researched or not. The identification of the error is therefore not a determination of a cause, but in any event, a type of classification with regard to a possibility of elimination of the respective error, and, better yet, serves merely for easing the communications among experts in the field.
The previously mentioned U.S. Patent Applications include methods which enable the determination of correction values or parameters for the print substrate conditional register mark errors, which make it possible to correct the printing process with regard to the register mark accuracy, thereby improving the same. For this purpose, the correction values obtained by respective calibration runs can be stored in a correction table based on the print substrate, and the substrate sheets to be printed can be corrected for the printing process by accessing this table.
However, it may also be provided that large-surface toner fields are placed on a conveyor belt for the substrate to be printed, especially in the larger spaces located between the print substrate sheets, which would fit into a print substrate sheet, but for which for whatever reason no print substrate sheet has been provided, for instance, in the currently running printing job, which means that a respective clearance remains. In other words, a window or frame would exist, however, for inserting a print substrate sheet in the control of the printing process. Such a window, or such a clearance is now being used in order to place a toner field onto the largely free conveyor belt, preferably in the size of a print substrate sheet, preferably corresponding to the size of a maximum sized sheet of, for example, 470 mm by 343 mm.
It is, in fact, a known problem that with electro-photographic printing, if a toner layout is fixed with a roller fixing device using fixing oil, oil can be carried off up to the area of the illustration station by a subsequent reverse printing, where it may lead to problems: because it dirties, for instance, a photoconductor, or a rubber blanket. By a large-surface toner field placement on the conveyor belt, such oil residue, for example, can be safely bound in the toner, and later removed.
Oil residue can thereby be prevented from reaching the printing layout of the print substrate sheet to be printed next, thus influencing the print layout in that area.